Examples that Illustrate Informed Substitution

Arrestin recruitment can also be compared between diiierent substitution point mutants of the same receptor. Such comparisons may yield information about key residues for arrestin recruitment, for example, the respective roles of specific phosphorylation sites in the receptor C-terminal (BusiUo et al., 2010 de Mimnik et al., 2015). Substitution of some of these potential phosphorylation sites, but not others, may result in the absence of detectable arrestin recruitment. One conundrum of such analyses is that amino acid substitutions in the receptor may alter the sensitivity of the BRET system via conformational effects. This is illustrated by an example in which BusiUo et al. foimd that alanine substitutions of phosphorylation sites in the CXCR4 C-terminal could lead to increased BB T efficiency—the authors concluded a conformational effect (BusiUo et al., 2010). Because these effects may also decrease (or abolish) BRET efficiency for purely technical reasons (and not as an intrinsic property of the mutant receptors), we recommend to gather complementary evidence before concluding on the efficacy of arrestin recruitment. Alternative interpretations such as the occurrence of different arrestin recruitment modalities to different receptor mutants should also be considered. This applies in particular to mutations of phosphorylation sites, as the intricate receptor phosphorylation patterns (bar-codes) by multiple kinases are known to determine the recruited arrestin species (Nobles et al., 2011). 

To illustrate the technique we will consider a few examples of free radicals which have been prepared in the rotating cryostat. In particular phenyl and acetyl radicals and methyl-substituted allyl radicals are of interest as they have not been trapped previously or identified with certainty. Since electron spin resonance has been used extensively to detect and identify the free radicals, account of the results will inevitably involve some description and analysis of their spectra, but we wish to focus the main discussion on the conclusions that can be drawn about structure and reactivity of the radicals. For information about the principles of e.s.r. and the interpretation of the spectra of free radicals the reader is referred to review articles and books on the subject (Symons, 1963 Norman and Gilbert, 1967 Maki, 1967 Horsfield, 1967 Carrington and McLachlan, 1967 Ayscough, 1967 Carrington and Luckhurst, 1968). 

Suffixes and endings are of many different kinds (Table III), each of which conveys specific information. The following examples illustrate particular uses. They may specify the degree of unsaturation of a parent compound in substitutive nomenclature hexane, hexene and phosphane, diphosphene, diphosphyne. Other endings indicate the nature of the charge carried by the whole compound cobaltafe refers to an anion. Further suffixes can indicate that a name refers to a group, as in hexyl. 

Two examples are mentioned here to illustrate how IR spectroscopy can be used to obtain structural information about SAMs The molecular structure of a thiol-substituted carotenoid (see Sect. 4.3.7) was investigated by IR spectroscopy. Comparison of the IR spectrum of the carotenoid in the KBr matrix with that in the SAM shows the peak frequencies to be slightly higher... 

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